The present application relates to an electrode in which an active material includes a lithium phosphate compound, to a secondary battery using the electrode, and to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as a mobile phone and a personal digital assistant (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their long life. Accordingly, as an electric power source for the electronic apparatuses, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed. In these days, it has been considered to apply such a secondary battery to various other applications in addition to the foregoing electronic apparatuses. Representative examples of such other applications may include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill.
Secondary batteries utilizing various charge and discharge principles to obtain a battery capacity have been proposed. In particular, a secondary battery utilizing insertion and extraction of an electrode reactant or a secondary battery utilizing precipitation and dissolution of an electrode reactant has attracted attention, since such a secondary battery provides higher energy density than lead batteries, nickel-cadmium batteries, and the like.
The secondary battery includes a cathode, an anode, and an electrolytic solution. The cathode contains an active material (cathode active material) related to a charge and discharge reaction. As the cathode active material, generally, a lithium composite oxide having a bedded salt-type crystal structure such as LiCoO2, LiNiO2, and LiMn2O4 is used to obtain a high capacity and a high voltage.
However, in the case where the lithium composite oxide is heated to a temperature from 200 deg C. to 300 deg C. both inclusive in a state of being charged, the lithium composite oxide easily extracts oxygen. Therefore, studies have been made to use a lithium phosphate compound instead of the lithium composite oxide taking safety into consideration. The lithium phosphate compound is LiFePO4 or the like having an olivine-type crystal structure.
In the lithium phosphate compound, however, for example, a sufficient battery capacity and/or the like tends to be less likely to be obtained, since an insertion and extraction reaction of an electrode reactant at the time of charge and discharge is slower and the electric resistance is higher compared to in the lithium composite oxide. Therefore, in order to improve battery characteristics of a secondary battery using the lithium phosphate compound, various studies have been made.
Specifically, in order to increase a charge and discharge capacity at the time of large-current charge and large-current discharge, electrically-conductive fine particles such as Ag are supported by surfaces of powders of a lithium-iron-phosphate-based material represented by general formula LizFe1-yXyPO4 (X represents Mg or the like, 0≦y≦0.3, and 0<z≦1), or a composite is configured of particles of a lithium-transition-metal composite oxide represented by general formula LiMePO4 (Me represents a divalent transition metal) and carbon substance fine particles (for example, see Japanese Unexamined Patent Application Publication Nos. 2001-110414 and 2003-036889). In order to obtain superior electron conductivity, a compound represented by general formula LixFePo4 (0≦x≦1) is mixed with a carbon material, and the primary particle diameter and the specific surface area of such a compound are defined (for example, see Japanese Unexamined Patent Application Publication No. 2002-110162). In order to improve discharge performance at the time of high-rate discharge, olivine-type lithium phosphate represented by general formula LiMPO4 (M represents Co or the like) and a binder (polyacrylonitrile: PAN) is used (for example, see Japanese Unexamined Patent Application Publication No. 2005-251554). In order to improve cycle characteristics and safety at the time of high-rate discharge, the porosity, the fine pore diameter, and the like are defined for a cathode containing lithium-transition-metal-phosphate represented by general formula LixMPO4 (M represents Co or the like, 0<x<1.3) and/or the like (for example, see Japanese Unexamined Patent Application Publication Nos. 2010-225366 and 2010-015904). In order to balance a high battery capacity and superior load characteristics, a relation between the average particle diameter of primary particles of a lithium phosphate compound and the void (fine pore diameter) between the primary particles is defined (for example, see Japanese Patent No. 4605287).